This invention relates to a method and apparatus for reducing the emission of nitrogen oxide to the atmosphere from stationary sources. More particularly, this invention relates to a method and apparatus for effecting such a reduction wherein a suitable reducing agent is employed without the aid of an extraneous catalyst.
As is well known, various oxides of nitrogen are produced during the combustion of most fuels with air. In general, these oxides result either from the oxidation of nitrogen in the air at the elevated temperatures of combustion or from the oxidation of nitrogen contained in the fuel. Such formation can, of course, occur in both catalytic and non-catalytic combustion although the formation is much more predominant in non-catalytic combustion. Various oxides of nitrogen are also produced in chemical processes such as in the production of nitric acid.
Generally, these oxides have been emitted to the atmosphere with the effluent gases from such combustion operations and chemical processes. As is also well known, certain of these oxides and particularly nitrogen dioxide cause deleterious effects in human, plant and other animal life. Moreover, certain of these oxides have been identified as reactants in photochemical smog formation. The continued emission of these oxides to the atmosphere, then, poses a threat to the health and welfare of the community and to the plant and animal life therein. As a result, considerable effort has been made recently to either significantly reduce and/or eliminate such emissions to the atmosphere both from internal combustion engines as well as stationary sources such as power plants, process furnaces, incinerators and the like -- all as a part of a program to improve and/or protect the environment.
As a part of this effort, several processes have heretofore been proposed for reducing the emission of such oxides to the atmosphere. These include various combustion modifications such as operations with low excess air, two-stage combustion wherein first stage combustion is accomplished with substoichiometric air, partial and complete catalytic combustion, combustion in the presence of various NO.sub.x inhibitors and catalytic and non-catalytic treatment of combustion effluent so as to effect either partial or complete reduction of the nitrogen oxides which might otherwise be emitted to the atmosphere. Each of these processes has, of course met with some degree of success and, indeed, any one could be an optimum procedure for a particular application depending upon such factors as the total quantity of NO.sub.x involved, the degree of reduction sought, and the nature of the combustion or chemical process involved. For the reduction of NO.sub.x emissions from stationary combustion sources such as furnaces and boilers, however, the catalytic and non-catalytic treatment of the combustion effluent appear to offer the greatest advantages. Moreover, economic considerations presently appear to favor the use of a non-catalytic treatment such as that disclosed in U.S. Pat. No. 3,900,554 granted Aug. 19, 1975, provided at least that the desired degree of NO.sub.x reduction can be achieved therewith at all furnace and/or boiler loadings. In this regard, it should be noted that due to the critical and relatively narrow temperature range over which effective NO.sub.x reduction can be accomplished, some difficulty has been encountered in effectively applying this process in all furnace and boiler operations, independent of the particular furnace or boiler type, and at all furnace and boiler loadings. Also, due to the relatively high temperature at which the ammonia is added some difficulty has been encountered in effecting and controlling the injection of the ammonia into the combustion effluent gas stream without some undesirable dissociation of the ammonia. In light of these deficiencies, the need for an improved process which would permit the use of a non-catalytic effluent treatment in any furnace and/or boiler and the realization of the advantages associated therewith is believed to be readily apparent.